Multilayered wiring units are advantageously used to support a multiplicity of telephone subscribers where space is at a premium. For example, in FIG. 1, there is shown a multilayered wiring unit 17 for connecting customer telephone equipment to a central office. Multilayered wiring unit 17 includes a bottom wiring layer 7 having a splice chamber and an array of protector devices mounted thereon. The splice chamber includes connectors for connecting a cable to the protectors. A top wiring layer 5 includes an array of customer bridges, each adapted for coupling to a particular subscriber line. The array of bridges is electrically coupled to the protector array through at least one removable electrical connecting cable 3. Top wiring layer 5 is pivotably attached to bottom wiring layer 7 by a hinged member 9 so that top wiring layer 5 may be opened to reveal bottom wiring layer 7. In this way, access to the protector devices is provided while keeping the total "footprint" of the wiring unit at a minimum. A more detailed description of such prior art multilayered wiring units is provided in U.S. Pat. No. 5,363,440, the entire contents of which is incorporated herein by reference.
One significant drawback of multilayered wiring unit 17 of the prior art is that connecting cable 3 traversing hinged member 9 is subject to stress and wear caused by the repeated opening and closing of top wiring layer 5. Top wiring layer 5 may be opened and closed hundreds of times by technicians requiring access to bottom wiring layer 7 to perform additional installations or system troubleshooting. As a result of these repeated openings and closings, excessive bending stress is placed on the portion of connecting cable 3 traversing hinged member 9 which may cause an electrical failure in connecting cable 3.
A test was performed to determine the amount of bending stress connecting cable 3 could withstand before failing as a result of repeated openings and closings. The test employed a test unit that opened and closed a simulated hinge portion of a multilayered wiring unit twelve times per minute. A typical twenty-five pair (50 conductor) cable was connected between a top wiring layer and a bottom wiring layer in a conventional manner. All the wires in the cable were attached in such a way as to create one single conductor connected in series between a light and a power supply so that if any wire of the fifty broke, the light would either flicker or go out. It was found that after 405 opening and closing cycles, the light went out, indicating that a wire in the cable electrically failed as a result of excessive bending stress. This high failure rate is unacceptable especially for heavily used multilayered wiring units.
Accordingly, a multilayered wiring unit in which there is reduced stress and wear on the connecting cable or individual conductors traversing the hinge member between the multiple layers is desirable.